MICROSCOPE. 
mate of the magnifying power of a microscope 
must depend upon the standard which we 
assume as the ordinary distance at which the 
object is seen with the naked eye; since, if 
brought within five inches, it is seen under 
double the angle, and therefore of double the 
size which it appears to possess at ten inches. 
Tf, therefore, the former distance were taken as 
astandard, the magnifying power of a lens or 
‘microscope will only be half that at which it 
‘would be estimated with reference to the other. 
Nearly all opticians, however, have agreed in 
‘considering ten inches as the standard of com- 
parison; and when, therefore, an object is 
figured as magnified 100 diameters, it is meant 
that this figure placed at ten inches distance 
from the eye is 100 times the dimension, each 
way, of the real object seen with the eye alone 
at a similar distance. The measurement of 
the magnifying power of simple or compound 
microscopes by this standard is attended with 
no difficulty. All that is requisite is to have 
a glass or thin slip of ivory accurately divided 
) a small fraction of an inch (;};th will usually 
answer very well), and a common foot-rule, 
divided to tenths of an inch. The glass or 
ivory micrometer being adjusted to the focus 
of the magnifier, the rule is held parallel with 
it, at the distance of ten inches from the eye. 
the second eye be then opened, whilst the 
other is looking at the object, the circle of light 
‘included within the field of view, and the 
object itself will be seen faintly projected upon 
_ the rule ; and it will be very easy to mark upon 
_ the latter the apparent distances of the divi- 
sions on the micrometer, and thence to ascer- 
| the magnifying power. Thus, supposing 
each of the divisions of ;,th of an inch 
sponded with 13 inch upon the rule, the 
linear magnifying power is 150 diameters; 
‘if it corresponded with half an inch, the mag- 
“nifying power would be 50 diameters. The 
erficial magnifying power is of course esti- 
mated by squaring the linear; but this isa 
mode of statement never adopted by scientific 
_ observers, although often employed to excite 
_ popular admiration or to attract customers by 
those whose interest is concerned in doing so. 
__When the magnifying powers of the several 
_ objectives of a microscope are known, it is easy 
_ to make a fair approximation to the real size of 
an object under examination, by projecting its 
_ Image, as before, upon a foot-scale held at ten 
_ Inches distance ; and the apparent dimensions 
it there exhibits, divided by the magnifying 
ig employed, will of course be its real size. 
_More accurate measurements are generally re- 
quired, however, although the foregoing may 
“serve as a sufficiently near approximation for 
_ ordinary purposes. Various methods of ob- 
taining these have been devised. The most 
perfect measurements are obtained by a fine 
_____micrometer-screw, by turning which either the 
_ object or the body will be made to execute a 
_ transverse movement; just as, in the fine ad- 
_ justment of the focus, the body is made to 
_ pproach or recede from the stage. This ap- 
_ paratus may be attached to the stage, if it be 
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thought preferable to move the object, or to 
the arm if the body be made to traverse. In 
proportion to the fineness of the screw, and to 
the size of the milled-head, will measurements 
be obtained of minute accuracy. In the focus 
of the eye-piece a very fine thread is placed ; 
and one edge of the image being brought 
against it, the position of the micrometer is 
noticed; and the other edge being then brought 
to the same line, the number of divisions of 
the micrometer-screw, which have passed over 
the index, will indicate its size. The expen- 
siveness of this micrometer, when made with 
sufficient accuracy and minuteness, is the only 
bar to its general employment. Other means 
have been devised, however, which are scarcely 
inferior; and the simplest of these requires 
only the use of an eye-piece of a construction 
different from the ordinary, with coarsely di- 
vided glasses. The micrometer eye-piece is 
made upon the principle of Ramsden’s. The 
optical part of it differs from that in common 
use, in having the plane side of the field-glass 
turned towards the object; and in the adjust- 
ment of the foci of its lenses in such a manner 
that the image to be viewed by it must be 
beneath the field-glass instead of being be- 
tween it and the eye-glass. In its focus there 
is placed a plane glass with divisions on it; 
these may be from ith to jth of an inch 
apart ; the latter will enable very minute mea- 
surements to be taken. By this arrangement, 
when the object is brought into focus, it ap- 
pears as if it were traversed by cross lines; 
since its image is coincident with the divided 
glass, and is, like it, viewed by the eye-piece 
as by a simple microscope. The value of these 
divisions will, of course, depend upon the 
degree in which the object is magnified in the 
image; and they must be ascertained for every 
objective, by the divided glass or ivory mi- 
crometer. This being brought into focus, and 
so placed that the direction of its two sets of 
lines shall correspond with those of the divided 
glass in the eye-piece, the number of divisions 
in the latter, corresponding to each division of 
the former, must be observed, and their value 
will be thus ascertained. Thus, if one divi- 
sion on the ;,th inch micrometer be found to 
coincide with eight on the eye-piece, these 
eight will together indicate a dimension of j;th 
of an inch upon the object; and each division 
of the eye-piece will of course be equivalent 
to the ith of an inch. If there is not an 
exact correspondence, or if it be desired to 
obtain a power which can be expressed in 
round numbers, a little alteration in the length 
of the body, made by drawing out his eye- 
piece, will enable the microscopist to effect 
this ; but the eye-piece should be marked, so 
as to be adjustible to the same point again, 
when the same magnifying power is employed, 
When the value of the divisions on the glass 
in the eye-piece is ascertained for every mag- 
nifier, the object-glass micrometer may be put 
aside altogether; since by the use of the eye- 
piece alone, a series of lines, the real distance 
corresponding to which is sae 4 projected 
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